Unfortunately most flowers will take longer than just a few weeks, but forcing paperwhite narcissus bulbs (need no chilling and take about a month to bloom) and starting amaryllis bulbs is always fun, as is growing any type of seed. You could grow grass seed in little gift baskets and wrap them with red ribbon; pot up chives and other herbs as windowsill gifts, start ivy or rosemary topiaries, and try growing radishes and lettuce for the holiday salad. You could also experiment with growing sprouts of various types -- a jar, a few seeds, and instructions make a nice gift. Have fun with the kids!